As operating speed of semiconductor memory devices increases, swing ranges of signals which are interfaced between the semiconductor memory devices become narrower in order to minimize delay time required for transferring the signals.
A unit memory cell of a dynamic random access memory (DRAM) device includes a transistor and a capacitor. The transistor performs a switching operation and the capacitor stores electric charges, i.e., data. The data have a logic level “HIGH” or a logic level “LOW” according to the electric charges stored in the capacitor.
Because of a characteristic of the capacitor, the electric charges are gradually reduced as time passes. Accordingly, a refresh operation which refreshes the stored data in the memory cell at every predetermined period is required to continuously maintain the stored data in the memory cell.
There is power consumption due to the refresh operation performed by a DRAM controller. Accordingly, in a battery operated system under a low power circumstance, it is important to reduce the power consumption due to the refresh operation.
One way to reduce the power consumption due to the refresh operation is to change a period of the refresh operation in response to temperature. In a DRAM device, a time period during which the data of the memory cells can be maintained increases as the temperature decreases. After dividing the temperature into a plurality of regions, if an operating frequency of the refresh operation is lower under a low temperature region, it is possible to reduce the power consumption. Accordingly, it is required to detect the temperature of the DRAM device to lower the operating frequency of the refresh operation.
In addition, the DRAM device generates increasing heat as integration levels and operating speeds are increased. This heat raises an internal temperature of the DRAM device so as to disturb a normal operation of the DRAM device, thereby causing a fault of the DRAM device. As a result, it is required to detect the temperature of the DRAM device to output the detected result.